Theiler's virus, a picornavirus, causes a persistent infection of the spinal cord of genetically susceptible mice that is accompanied by chronic infection and primary demyelination. As such, it provides one of the best animal models for immune-mediated demyelination. Theiler's virus first infects neurons, but later persists in oligodendrocytes and macrophage/macroglial cells. This shift from gray to white matter coincides with the appearance of virus-specific immune responses, suggesting that immune pressure keeps the virus out of neurons but is unable to clear it from glial cells. In this application, we propose to test a model in which the virus traffics sequentially from neurons to oligodendrocytes to macrophages, with oligodendrocytes providing an obligatory intermediate in the face of immune pressure. First, we will monitor the time course of Theiler's infection of various cell types in wild-type C3H and in shiverer and rumpshaker mice, which have different severe myelin defects and are resistant to persistent infection. We will examine whether oligodendrocytes, macrophages or both are never infected, or infected only transiently, in the resistant mutant mice. Bone marrow transfer experiments will reveal whether CNS- or bone marrow-derived cells from shiverer and rumpshaker mice confer resistance to persistent infection and in vitro culture of oligodendrocytes and macrophages will reveal whether the intrinsic permissiveness of these cells to viral infection is altered by the mutations. Second, we will determine whether myelin is the portal of entry of the virus into the white matter. We will express viral genomes specifically in neurons in mixed cultures of oligodendrocytes and neurons, and track viral spread into oligodendrocytes in the presence and absence of successful myelination. Experiments performed with female rumpshaker heterozygotes will afford us the possibility to work with natural chimeras in vivo, because the rumpshaker mutation is X-linked. Finally, we will use in vivo labeling methods to monitor the migration of blood-borne macrophages into inflammatory CNS lesions, and determine the frequency with which these newly arrived cells become infected. This will test the possibility that persistence is achieved, in part, by the chronic inflammation that ensures the existence of a renewable host population. Together, this model may prove a paradigm for the functional transfer of infectious material from neurons to oligodendrocytes, and from thence to cells of the immune system.